The present inventive concept is related to generator architectures and in particular to generator architectures utilizing main field rotating power converters.
In the simplest terms, generators convert mechanical energy to electrical energy via the interaction of rotating magnetic fields and coils of wire. A multitude of generator architectures have been developed with various means of providing interaction between magnetic fields and coils of wire. For example, a permanent magnet generator (PMG) utilizes permanent magnets to generate a constant magnetic field, which is rotated via the mechanical energy supplied by a prime mover such that the rotating magnetic field interacts with the stator coils to provide an output voltage. Another type of generator supplies current through a coil to generate the desired magnetic field, which is rotated via the mechanical energy supplied by a prime mover, such that a rotating magnetic field is created that interacts with stator coils to provide an output voltage.
In the former example, the output voltage supplied by the PMG depends only on the magnitude of the mechanical energy supplied by the prime mover. In the latter example, the output voltage of the generator can be regulated by varying the current supplied to the field coil. For applications in which the output voltage must be regulated, the latter example, known as a wound field synchronous machine, is widely utilized. A PMG is sometimes utilized in conjunction with the wound field synchronous machine to source the current supplied to an exciter field winding to regulate the output of the brushless wound field synchronous machine.
For example, in aircraft applications, a typical variable frequency generator (VFG) includes a permanent magnet section, an exciter section, and a main generator section. The permanent magnet portion includes permanent magnets employed on the rotating portion, which generate an alternating current voltage on the stator portion. The AC voltage provided by the permanent magnet portion is rectified and selectively applied to the exciter field winding on the stationary portion of the exciter. The exciter field current interacts with the rotating exciter armature windings to provide AC voltage. A rotating rectifier rectifies the AC voltage and supplies the DC voltage to a main field winding on the rotating portion of the main generator section. Rotation of the motive power shaft and the main field winding induces three-phase AC output voltage on the main generator armature winding. The magnitude of the AC generator output voltage is regulated by controlling the current supplied to the exciter coil on the stationary portion of the exciter. On characteristic of this architecture is that the output of the generator may be a function of the rotational speed of the generator and load. In cases with a belt-type interface between prime mover shaft and the generator shaft sudden load changes may also result in sudden changes in generator speed. As a result, sudden variations of the generator output voltage, such as sudden voltage increases, may occur. There is a need to improve voltage regulation. There is also a need to simplify control by replacing a field regulated exciter with an unregulated PMG-based exciter.